Toward realizing a next generation network (NGN), establishment of a large-capacity photonics network with increased transmission speed and dense wavelength multiplexing is in progress. Concurrently, the introduction of a high-speed optical transmission system has gathered momentum.
A broadband and large-amplitude driver IC dedicated to drive an optical modulator is necessary to establish a high-speed optical transmission system. For example, in order to realize high-speed optical modulation of 40 G bits/second in an optical modulator using LiNbO3 (LN modulator), a driver circuit (driver IC) is required to operate in a broadband and to output a large-amplitude voltage of about 4 to 5 Vp-p to obtain a required electro-optic effect.
A feedback amplifier circuit has conventionally been known as a broadband amplifier circuit. For example, Japanese Laid-open Patent Publication No. 10-247831 describes a differential configuration using a pair of feedback amplifier circuits.
FIG. 14 illustrates a configuration of a main part of the amplifier circuit described in Japanese Laid-open Patent Publication No. 10-247831.
The amplifier circuit in FIG. 14 has a differential configuration including two feedback amplifier circuits, each including a two-stage emitter-grounded amplifier circuit 10 having an input-stage transistor 11, an output-stage transistor 12, and a load resistor RL; and a feedback circuit 20A having a feedback transistor 21 and a feedback resistor RF.
In the above-described feedback amplifier circuit, a signal having a frequency characteristic degraded by amplification in two stages is fed back to an input of a second amplifying stage, so that an eventual high-frequency component can be increased and a broadband frequency characteristic can be ensured.
As described above, the driver IC to drive an optical modulator requires not only a broad bandwidth but also a large amplitude. Accordingly, the amplitude of the output of the above-described conventional feedback amplifier circuit may be increased, which may be applied to the driver IC.
However, in the case of increasing the amplitude of the output of the above-described conventional feedback amplifier circuit, the following problem arises.
In the conventional configuration, the feedback resistor RF is set to a relatively small value so that a broadband frequency characteristic can be ensured, and thus the difference between the potential on the collector side of the output-stage transistor 12 (point B in FIG. 14) and the potential on the base side (point A in FIG. 14) is small. In other words, an operating-point potential of output is low (on the side of low potential VEE). Thus, even if an effort is made to increase the amplitude of the output, an increase in amplitude on the lower side is limited.
For this reason, in order to enable a large-amplitude output operation in the conventional configuration, it is necessary to increase the potential difference between the collector side (point B) and the base side (point A) of the output-stage transistor 12 (i.e., raise the operating-point potential of output) so that the increase in amplitude on the lower side is not limited.
In order to increase the potential difference between the collector side (point B) and the base side (point A) of the output-stage transistor 12, the value of the feedback resistor RF or the value of a drive current (control current) I may be increased.
However, if the value of the feedback resistor RF is increased, the frequency characteristic at point A degrades. On the other hand, if the value of the drive current I is increased, the size of the feedback transistor 21 should be increased. In that case, a stray capacitance CCB between the collector and base of the feedback transistor 21, which is parallel to the load resistor RL illustrated in an equivalent circuit diagram in FIG. 15, becomes large, so that the frequency characteristic at point B degrades.
As described above, in the amplifier circuit having the conventional configuration, it is difficult to ensure both a broadband frequency characteristic and a large-amplitude output.